It is common experience that holes bored using single point boring tools are sized after the fact, which is merely to acknowledge that it has heretofore been impossible to set the cutting point of a boring tool in such a way as to pre-size the resulting hole accurately.
In any boring operation there are typically a number of essentially indeterminate factors which play a part in determining the size of the hole produced, irrespective of the machinist's skill in setting the effective cutting radius of the tool. Prominent factors include the deflection of the boring bar under load, and the deflection or spring back of the workpiece itself, both of which may vary with the material and hardness of the workpiece, with the velocity and depth of cut, and with the feed rate of the boring-tool. High-speed, high-load boring operations also experience the dimensional effects of elevated temperatures in both the tool and the work, and the centrifugal forces acting on the tool, particularly on longer tool bars. In repetitive operations, the wear on the tool point also becomes a factor.
In practice, therefore, accurate boring, i.e., boring to close tolerances, has been a process of cut and try, out of which boring machine operators evolved the practice of taking a fairly heavy rough cut to remove, say, 80 percent of the metal to be removed in finally sizing the hole, whether a previously unmachined cored bore in a casting or a pre-drilled hole in a steel weldment or the like, followed by a semi-finish cut aimed at removing another 15 percent of the material. Experience with the behavior of a given tool, whether cantilevered from the spindle or supported as an arbor in an outrigger bearing, afforded the operator little more than a basis for predicting a probable result which had to be verified after the semi-finish cut by measurement of the bore before proceeding further.
This process of cut and measure was then repeated, again based upon the experience and skill of the machinist in predicting the behavior of the tool and the workpiece in determining the amount of metal that could safely be removed in a third and subsequent cuts, in which it was not possible to predetermine with accuracy the precise radial position of the tool point while rotating, nor its radial deflection under load.
Precision boring operations in these circumstances required consummate skill on the part of the operator in determining the amount of metal which could be removed in a given pass of the single-point tool through the workpiece, while the necessity of approaching a final result by decreasing increments inevitably left the operator with a choice between too large or too small, or, chance aside, multiplied drastically the machining time as the machinist engaged in a progressive reduction of the depth of successive cuts to hold tolerance.
This situation, to the extent dependent upon knowing the precise radial location of the rotating cutting tip of the boring tool, is magnified in the unmanned, more-or-less universal machine tools called "machining centers", i.e., tools which are adapted for milling, drilling, tapping, boring, counterboring, and spot facing, according to the nature of the particular tool mounted in the machine spindle. A complex total machining operation of that kind necessarily involves the automatic change of tools from operation to operation, with each tool presented successively to the spindle of the machine, in which it is typically centered by the seating of a taper on the tool holder in a corresponding taper in the spindle, and where a tool gripper associated with the spindle engages a knob on the tool holder to draw the tool into secure driving engagement with the spindle.
The automatic transfer of tools between the spindle of the machine and the storage pockets of an automatic tool changer or back-up tool reserve inevitably exposes the seating tapers of the tool holders to wear and tear, as well as to the accumulation of foreign material which interferes with the accurate centering of the tool holder as it is drawn tightly into the tapered seat of the spindle. In these circumstances, the accuracy of any pre-setting of the tool point of a boring tool is illusory, and the actual radial location of the tool point is difficult, if not impossible, to determine except from its effect upon the workpiece after the fact.
Rotary cutting tools of almost any nature which are coupled with the machine spindle in the manner described are susceptible to the same difficulty of achieving precise coaxial alignment with the machine spindle when considered in the context of maintaining close tolerances in a finish cut. In the case of a milling cutter, any deviation from precise alignment coaxially with the machine spindle will result in runout of the cutter, which, as a result of its eccentricity with the spindle, will cut to a depth, or in the case of an end-mill, to a shoulder as well as depth, which will deviate from that designed into the operation by the programmed movement of the machine spindle along and transversely of its own axis.
Touch probe systems are available for determining the axial or radial location or dimension of the tip of a boring tool or of the cutting path of a milling cutter under static conditions, i.e., by presenting the non-rotating spindle-mounted tool to a so-called tool datumming touch probe such as that available from Reneshaw, Inc., but such measurements performed with the tool at rest are unable to determine the location and effective radius of the cutting tool under the effect of centrifugal forces, which can be substantial at higher cutting speeds and of magnified effect in tools of substantial axial length, and, in the case of a radially adjustable boring bar, at the upper radial limits of tool adjustment.
It is an object of this invention to overcome much of the aforedescribed limitation upon the metal cutting art by providing an apparatus and method for unmanned determination of the exact radial extent and axial location of the cutting path of a rotating cutting tool so as to eliminate that major indeterminate factor from automated machining operation.